This section should give you some general information
about serial ports. If you do not find what you want
here, check into the Terminal and Dialup sections of the
handbook.

The ttydX (or cuaaX) device is the regular
device you will want to open for your applications. When
a process opens the device, it will have a default set of
terminal I/O settings. You can see these settings with
the command

#stty -a -f /dev/ttyd1

When you change the settings to this device, the settings
are in effect until the device is closed. When it is
reopened, it goes back to the default set. To make
changes to the default set, you can open and adjust the
settings of the ``initial state'' device. For example, to
turn on CLOCAL mode, 8 bits,
and XON/XOFF flow control by
default for ttyd5, do:

#stty -f /dev/ttyid5 clocal cs8 ixon ixoff

A good place to do this is in
/etc/rc.serial. Now, an application will have these
settings by default when it opens
ttyd5. It can still change these settings to its
liking, though.

You can also prevent certain settings from being changed
by an application by making adjustments to the ``lock
state'' device. For example, to lock the speed of ttyd5 to 57600 bps, do

#stty -f /dev/ttyld5 57600

Now, an application that opens
ttyd5 and tries to change the speed of the port will
be stuck with 57600 bps.

Naturally, you should make the initial state and lock
state devices writable only by
root. The MAKEDEV script
does not do this when it creates
the device entries.